journal of acoustic emission 2017

160 Pages·2017·15.49 MB·English

JOURNAL OF
ACOUSTIC EMISSION
2017
Volume 34
(First Installment)
Editors
M.A. Hamstad and G. Manthei
Published by
Acoustic Emission Group, Encino, CA USA
JOURNAL OF ACOUSTIC EMISSION
Contents
Volume 34, 2017
34-001 Through-Transmission Characteristics of AE Sensor Couplants
Kanji Ono
34-012 An Experimental Study of Acoustic Emission Waveguides
Kanji Ono
34-052 Clustering of Fiber-Break Related Events in Carbon Fiber Reinforced Polymer
Composites Using Acoustic Emission
T. Whitlow and M. Sundaresan,
Selected Papers from PROGRESS IN ACOUSTIC EMISSION XVIII (2016)
Review of Acoustic Emission Source Mechanisms on Large Movable Structures, David E.
Kosnik, S01-S09
On Acoustic Emission Sensor Characterization, Kanji Ono, S10-S19
Classification of acoustic emission signals using wavelets and Random Forests: application to
localized corrosion, N. Morizet, N. Godin, J. Tang, M. Fregonese, B. Normand, S20-S25
Damage Evaluation of RC Bridge Deck under Wheel Loading Test by Means of AE
Tomography, T. Nishida, T. Shiotani, H. Asaue, T. Maeshima and Y. Kobayashi, S26-S31
Source of AEs from IG-SCC of Face Centered Cubic Metals under Static or Dynamic Straining,
Mikio Takemoto, Shuichi Ueno and Motoaki Nakamura, S32-S37
Detection of Acoustic Emission Signals with the Fabry-Perot Interferometer Type Optical Fiber
Sensor
Kazuki Tada1) and Hironobu Yuki, S38-S41
Remote imaging of plate-like structures with E-camera, Takahiro Hayashi, S42-S45
High-precision source location of AE event using automatic error correction of signal rising
time, Kaita Ito and Manabu Enoki, S46-S49
Influence of pre-compression on tensile behavior in wrought AZ31 studied by the acoustic
emission technique, Daria Drozdenko, Patrik Dobro, Jan Bohlen, Sangbong Yi and František
Chmelík, S50-S53
i
Visualization of internal damage in RC slab with single side access attenuation tomography,
Clément Granier, Tomoki Shiotani, Katsufumi Hashimoto and Takahiro Nishida, S54-S58
Acoustic Emission modeling from the source to the detected signal: model validation and
identification of relevant descriptors, Thomas Le Gall, Nathalie Godin, Thomas Monnier,
Claudio Fusco and Zeina Hamam, S59-S64
Evaluation of Irrigation Response using AE Method for Management of High-Frequency
Irrigation of Hydroponic Miniature-Tomato, Kensuke Kageyama and Takenobu Sakai, S65-S70
In-situ investigation of deformation mechanisms in various magnesium alloys by X-ray
diffraction and acoustic emission, Kristián Máthis, Gerardo Garcés, Jan Čapek and Alexei
Vinogradov, S71-S77
Acoustic emission and damage evolution at mean temperature under air of a SiC/[Si-B-C]
composite subjected to cyclic and static loading : towards lifetime prediction, E. Racie, N.
Godin, P. Reynard, M. R'Milli and G. Fantozzi, S78-S84.
Contents of the Proc. of IAES XXIII
ii
Through-Transmission Characteristics of AE Sensor Couplants
Kanji Ono
Department of Materials Science and Engineering,
University of California, Los Angeles (UCLA),
Los Angeles, CA 90095
Abstract
This study examined the behavior of AE sensor couplants for through-transmission of acoustic
signals or out-of-plane motion. In order to provide controlled conditions, face-to-face transmitter-
receiver arrangement was utilized. All liquid and gel couplants are found to give satisfactory
service with careful installation, producing the minimum couplant thickness of 5 to 8 µm. Their
performance is nearly identical to 1.2 MHz. With normal installation procedures, however, the loss
can be 3 to 5 dB with viscous resins and gels, as couplant thickness can become 10 to 15 µm and
high frequency components are lost more than the peak amplitude reduction. Honey turns out to
be an excellent couplant because of its higher acoustic impedance, while commonly used silicone
grease can be a poor choice at higher frequencies. Higher frequency transmission loss increases
with thickness as predicted by theory of through-gap transmission with approximately linear
dependence on frequency.
Keywords: Sensors, Couplants, Through-transmission, Attenuation, Frequency effects
Introduction
In all acoustic emission (AE) tests, a sensor must be mounted on a test object unless a more
expensive non-contact sensor is used. Sensor couplants have been discussed in several articles over
the years [1-6]. These were preceded by discussion of ultrasonic couplants for contact testing [7-
10]. ASTM E650 provides comprehensive guidelines for AE sensor mounting [11]. In selecting a
specific type of AE sensor couplant, Theobald [12] provided the best available guidelines at
present and summarized the features of various couplants. His group also showed some spectral
transmission characteristics of couplants [5].
We have recently evaluated sensor characteristics and assessed couplant behavior as a part of
the sensor studies [13]. Here, our studies are limited to the conditions for through transmission of
acoustic signals or out-of-plane motion, using face-to-face transmitter-receiver arrangements.
Quite often, AE applications detect in-plane motion of guided waves. The present results ignore
such AE signals since their detection critically relies on the sensor size, signal frequency spectra
as well as the vibration modes. The use of controlled condition also precludes effects of surface
roughness and curvatures, which often add to coupling loss. While most liquid couplants provide
satisfactory results with careful usage, conventional mounting practices can lead to loss of
sensitivities, especially at higher frequencies above 1 MHz. Results are given below.
Experimental
Couplants are characterized using face-to-face sensor arrangements. Both wideband and
resonant sensors are utilized. Most sensors have an almina face plate (except Dunegan S140B and
AET AC175L with epoxy facing) and were used in cleaned state without any special surface
J. Acoustic Emission, 34 (2017) 1 ©2017 Acoustic Emission Group
preparation. Transmitter is excited using short (<2 µs duration) high-voltage pulses. Received
signals are digitized at 2 ns interval and analyzed using FFT with using the data length of 262,144,
providing 1.9 kHz resolution. The signal length was mostly 10 µs (5,000 points) for broadband
sensor pairs and up to 100 µs (50,000 points) with a resonant sensor. The FFT routine of Noesis
(Enviroacoustics, ver. 5.8) was used with the pre-set parameters and Hamming window. All tests
were conducted at 21±1˚C.

Following couplants are examined. Water, motor oil (SAE 5-30 grade), honey, Vaseline, High
vacuum (HV) silicone grease (Dow Corning), Stopcock silicone grease (Dow Corning), Nonaq
stopcock grease (S-530, Fisher Sci.), Couplant resin (SC-6, AET), epoxy resin (quick-setting, HFT
or Harbor Freight Tools), ultrasonic shear gel (54-T04, Sonotech) and a plastic film as dry couplant
(Saran wrap). Of these, Nonaq and AET resin are no longer available commercially.

Results and Discussion

1. Best practice

When the viscosity of couplants is high, it takes time to establish ideal couplant thickness.
During mounting with such couplants, a sensor must be pressed using 20-50 N force and be rotated
to squeeze out excess couplant. With Vaseline and honey, it takes 5 to 10 min, while more viscous
(like SC-6) and stiffer (HV grease) couplants require 20 to 30 min. In contrast, water and motor
oil provide the maximum transmission from start, though these thinner couplants require some
pressure (several N force) to maintain good contact.

Fig. 1 FFT magnitude spectra of received signals through couplant, or t vs. frequency, f. Vaseline:
dark blue curve, motor oil: red, Nonaq: blue, High vacuum grease: purple, Stopcock grease: brown,
SC-6 resin: green. V103 transmitter and FC500 receiver.

Using a 1-MHz transducer (13-mm diameter, Olympus V103) as a transmitter and a wideband
sensor (FC500, AET) as a receiver in a face-to-face sensor arrangement, the output frequency
spectra are shown in Fig. 1. These also represent transmission spectra, t. Six couplants are used
and best contact was achieved for each. Data spread is less than 1 dB to 1.2 MHz, but three curves
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start to deviate lower. Differences between the spectra or ∆t are plotted in Fig. 2, where Vaseline
data is used as reference. Two curves for oil (red) and Nonaq (blue) deviate positively >1.2 MHz,
while three become lower. Of the three, the purple curve for HV silicone grease is the lowest,
being ~2 dB lower than Vaseline >1.5 MHz. This is surprising as this is a commonly used couplant
and the peak values differ only 0.5 dB (Vaseline result is higher than HV silicone). Another
silicone grease is comparable to viscous resin couplant (SC-6) with both ~1 dB lower than Vaseline.

Fig. 2 Differences in transmission spectra, ∆t, of received signals through couplant using Vaseline
spectrum as reference. Motor oil: red, Nonaq: blue, High vacuum grease: purple, Stopcock grease:
brown, SC-6 resin: green. V103 transmitter and FC500 receiver.

Fig. 3 Differences in transmission spectra, ∆t, of received signals through couplant using Vaseline
spectrum as reference up to 4 MHz. Motor oil: red, Nonaq: blue, High vacuum grease: purple,
Stopcock grease: brown. NDT Systems C16 transmitter and FC500 receiver.

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Another set of best-effort coupling used a 2.25-MHz transducer (13-mm diameter, NDT
Systems C16) as a transmitter and a wideband sensor (FC500, AET) as a receiver. In terms of peak
voltage of the waveform, HV grease was 1.8 dB lower and stockcock grease was 1 dB lower
(despite waiting >30 min under 15 N force and 50 N during measurement). Figure 3 shows the
differences in the spectra, again using the Vaseline result as reference. High frequency loss for the
silicone grease cases was even higher than in the previous tests, reaching 3 to 4 dB >2.5 MHz.
Both oil and Nonaq had better coupling than Vaseline above 2.5 MHz (these used 50 N force). It
is unclear why better coupling could not be made for the silicone grease cases, but at least these
showed lower peak voltages, indicating poorer performance.

In the best coupling conditions, all the couplants tested performed equally to 1.2 MHz, while
some have 1 to 2 dB (or 3 – 4 dB) higher transmission loss >1.5 MHz (or >2.5 MHz).

Fig. 4 Differences in transmission spectra of received signals, ∆t, through couplant using Vaseline
spectrum as reference. Non-optimum sensor installation for viscous couplants. Motor oil: red,
Nonaq: blue, Shear gel: dark blue, High vacuum grease: purple, Stopcock grease: brown, SC-6
resin: green, plastic film: black. V103 transmitter and FC500 receiver.

2. Normal coupling

In normal AE testing, we typically mount a sensor within a short time, spending no more than
30 sec. Adequate couplant is applied, squeezed with force in compression and in shear, then fix
the sensor with a spring and/or a holder, or by winding tape over the sensor. With such a method,
viscous couplants typically produce a thicker layer with higher transmission loss. In the next series
of tests, eight couplants were used. With the best Vaseline coupling as reference, spectral
differences are plotted in Fig. 4. Nonaq (blue), water (red dash) and shear gel (dark blue) achieved
essentially same peak amplitude (–0.1 to –0.6 dB) and the spectra differed less than 1 dB to 2 MHz
(except water had a slightly larger loss at 200-600 kHz). Silicone grease (brown and purple curves)
and SC-6 (green) suffered higher losses. In the case of HV grease, the loss reached 10 dB at
>1.7 MHz. These three had lower peak amplitude: HV grease showing –6.3 dB, stopcock grease,
–3.6 dB and SC-6, –2 dB, respectively. Another one is a plastic film, used as dry couplant. As
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expected, this performed poorly: –7 dB in peak signal amplitude and larger transmission loss
than most liquid/gel couplants except HV grease >1.2 MHz.

Thus, we need to be selective in choosing a couplant when high frequency response is sought.
In particular, the use of high viscosity couplants should be avoided unless one can spend adequate
time to properly achieve good coupling. One remedy is to raise the couplant temperature during
installation, thus lowering their viscosity. Another point of interest is that the shear gel did not
provide any special advantages even though it is expensive. Note also that Nonaq is no longer
marketed commercially. Our bottle is ~50 years old as we rarely used it.

3. Couplant vs. solid bonding

For mechanical stability, solid bonds are superior to liquid couplants. Bonding with glue is
also needed for better shear wave detection. It is assumed intuitively that solid bonds should
provide better wave transmission as the wave velocity increases with curing of thermosetting
polymers, such as epoxy. We examined the wave transmission of epoxy resin during curing. A
quick-setting 2-part epoxy (Harbor Freight Tools) was used. Three sets of curing tests were
conducted; two were face-to-face tests using resonant sensors (R15, S140B and AC175) and one
used 38-mm thick Al plate buffer between FC500 and 1-MHz UT transducer (Automation Ind.,
13-mm diameter). In all cases, the pairs were excited with high voltage pulse and the output
waveforms recorded and analyzed.

Fig. 5 Transmission spectra, t, of received signals through couplant. Vaseline reference: blue curve,
epoxy resin (as mixed): green, epoxy resin (cured for 30 min): red. FC500 transmitter and
Automation Ind. 1 MHz-1/2”.

The peak amplitude of the received signals remained constant from the time of coupling with
mixed epoxy resin to complete curing at 30 min. Some spectral changes can be seen in Fig. 5, but
overall spectra remain essentially constant. This data is for the higher frequency transducer pair,
but similar results are found for the pairs of resonant sensors. It is concluded that coupling behavior
is unaffected by epoxy curing, changing liquid resin into solid bond.

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The solid epoxy bond was stripped from the Al plate after test and its thickness was 13 µm.
The epoxy resins, both before and after mixing, had consistency of honey. From some epoxy
literature, quick setting types have viscosity of about 10 Pa·s (or 10,000 centipoise). Honey is
listed as 7 to 10 Pa·s at 20˚C. Thus, we can assume the thickness of moderately viscous liquid
couplant layers to be similar to the hardened epoxy bond, or 13 µm. Note that water has the
viscosity of 1 mPa·s and grade-30 motor oil 250 mPa·s. Their couplant layer thickness should be
less.

4. Couplant thickness

Next, we examined effects of couplant thickness. In comparing various couplants (cf. Fig. 4),
we used Vaseline as reference. Upon the initial application of this couplant, the thickness is much
higher than the minimum we can obtain after applying forces for 5 to 10 min. The received
amplitude is also lower. In the same series of tests for Fig. 4, Vaseline gives only 1.89 V, which
eventually increases to 2.91 V; i.e., the initial value is 3.7 dB lower. For these two tests, the
received signal spectra and their difference are plotted against frequency in Fig. 6. The difference
linearly increases with frequency, reaching 6.2 dB at 2 MHz. Thus, the loss in peak amplitude is
due mainly to the high frequency components above 500 kHz.

Fig. 6 Transmission spectra, t, of received signals through Vaseline couplant. Vaseline reference:
red curve, Vaseline couplant after normal mounting: blue. Spectral difference, ∆t, between normal
and reference installation is plotted in purple curve, which follows linear frequency dependence in
black dash line. ∆t (dB) = 3.10 f (MHz). FC500 transmitter and Olympus V104 receiver.

Using plastic shims with a 13-mm diameter hole, we examined the thickness effect directly
using honey, water, Vaseline and stopcock grease. Peak output voltage is plotted against the
couplant thickness (in mm) in Fig. 7. From the top curve for honey (purple), water (red), Vaseline
(blue) and stopcock grease data are shown. The lower three data follow straight lines in log-log
plotting, indicating the power law behavior. The bottom two lines are nearly identical. The honey
data can be represented by a second-order polynomial. The data show that the peak amplitude
decreases with thickness, but the thickness dependence is not unique. Since honey shows the
highest output below 0.2 mm, the viscosity is not the only factor for the transmission loss. A
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possible cause of honey’s higher response is its higher acoustic impedance, Z. It is at 2.89 E+6
kg/s m2 and is about twice that of water. The sound velocity for Vaseline and stopcock grease were
not found in the literature, so approximate measurements were made. With their published density
of 0.9 and 1.0, their acoustic impedances are found to be 1.72 and 1.87E+6 kg/s m2, which are 15
and 25% higher than that of water. With similar Z values, more steep decrease of signal amplitude
in Vaseline and silicone grease appears to come from their higher viscosity. In fact, it is more
appropriate to call them gels.

Fig. 7 Thickness dependence of output voltage of received signals through honey (dark blue),
water (green), Vaseline (blue), stopcock grease (red). FC500 transmitter and Olympus V104
receiver.

In Fig. 7, the left-most point for each curve corresponds to the highest output signal level and
the couplant thickness was extrapolated from the best fit curve. For water, this thickness was 5 µm,
while the thickness for the other three was 6 µm. These represent effective minimum couplant
thickness.

From the power law fit for Vaseline, the couplant thickness for the lower amplitude case in
Fig. 6 is estimated to be 14.6 µm. This is comparable to the case of epoxy discussed earlier. Thus,
gels like Vaseline and silicone grease can easily develop couplant layers two to three times that of
their best condition with attendant several dB of transmission loss. The situation also applies to
viscous liquids like resins and honey except honey has much better transmission even near 100
µm thickness.

While the data in Fig. 7 indicates overall transmission loss, frequency plays a major role in
determining the loss. Figure 8 shows the frequency dependence for two couplants, water and honey.
In both cases, the loss increases with frequency, roughly linear with frequency, but usually with
non-zero intercept. For water with the thickness of 0.04 and 0.13 mm, it reaches 4 and 12 dB at 1
MHz. For honey with 0.05 and 0.28 mm, the corresponding values are 2 and 10 dB at 1 MHz. That
is, water has approximately twice attenuation over honey and the same thickness or similar
attenuation at half the thickness. This effect is more pronounced at lower frequencies.
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